Membrane module

ABSTRACT

The invention relates to a membrane module for a device used to separate substance mixtures in a cross current method. Said membrane module comprises, a number of tubular membranes ( 22, 24, 26 ) which encircle, at least in sections, a common axis in an essentially helicoidal manner and which are provided for forming an essentially discoidal module ( 20 ). The aim of the invention is to improve a module of this type. To this end, at least two of the tubular membranes ( 22, 24, 26 ) encircle a common spiral axis in a helical manner, said membranes being preferably twisted like a cable, at least along the progression of a part of their essentially helicoidal encircling in order to form a tubular membrane strand.

[0001] The invention relates to a membrane module for a device forseparating substance mixtures in the crossflow process with a pluralityof tube membranes which, at least in certain portions, run substantiallyspirally around a common axis to form a substantially disk shapedmodule.

[0002] Crossflow separating processes operating with membrane modules ofthis type are used for example for clarifying fruit juices. For thispurpose, the unclarified fruit juice is introduced via a feed line intothe tube membranes, in the course of which part of the fruit juicehaving substances of small particle size (down to molecular size), knownas the permeate, penetrates the walls of the tube membranes and passesinto a permeate tank, while part of the fruit juice having solidsubstances of large particle size, the retentate, is discharged via adischarge line from the tube membranes and passed on for furtherprocessing. In the case of such separation or clarification of the fruitjuice, clogging of the walls of the tube membrane acting as filters canbe avoided by solid substances that are deposited on the tube wallsbeing detached by the fruit juice flowing through the tube membranes andremoved again from the tube membranes.

[0003] To obtain a satisfactory filtering performance of the membranemodules used in the crossflow separating processes described, acomparatively large filter area must be provided. Furthermore, it mustbe ensured that the tube membranes have a sufficiently small insidediameter to obtain a sufficiently small ratio of the interior volume ofthe tube membranes to the effective filter area that is required toobtain a satisfactory filtering performance. In this case, asatisfactory filtering performance can only be achieved when a pluralityof tube membranes are flowed through in parallel. To obtain a compactconstruction of corresponding separating devices that is required forpractical use,it is proposed in WO 98/19778 to use curved tube membranesinstead of straight tube membranes. In the case of a first embodiment ofthe separating devices described in the cited document, tube membraneswhich run helically around a common helix axis are used. To avoid damageto the tube membranes, it is proposed in this context to combine theindividual tube membranes running around the common helix axis intoclusters in which individual tube membranes are twisted in the manner ofa rope.

[0004] In the case of a further embodiment of the separating devicesdescribed in the cited document, the individual tube membranes formmembrane modules of the type described at the beginning in which theindividual tube membranes run spirally around a common axis to formflat, disk-shaped modules. Membrane modules of this type can be used inparticular whenever separating devices of a flat form are required. Forthis purpose, to obtain a sufficient filtering performance per membranemodule, a plurality of tube membranes may be wound spirally into one ormore layers parallel to one another.

[0005] However, when producing the membrane modules last described, ithas been found that the membrane modules obtained using conventionalwinding devices deviate from the desired shape in many cases, becausethere are great problems in guiding the individual tube membranes in thewinding device. This is attributable to the fact that the diameter ofthe finished membrane modules is particularly great in comparison withthe axial length, so that even small tensile or compressive stresses inthe axial direction of the modules cause individual tubes to depart fromthe winding plane. For this reason, it is generally required whenproducing the membrane modules described at the beginning for theindividual tube membranes to be wound by hand or for particularlycomplex control techniques to be used. In addition, it is generally alsonecessary for a radially extending adhesive attachment to be performedbetween the individual tube membranes at several points within theindividual membrane modules during the winding operation. Aftercompletion of the known membrane modules of the type described above, itis additionally also necessary for these membrane modules to be handledespecially carefully, in order that the individual tube membranes arenot displaced from their predetermined position.

[0006] In view of the problems of the prior art explained above, theinvention is based on the object of providing a membrane module of thetype described at the beginning which can be produced automaticallywithout especially complex control techniques and has satisfactorydimensional stability.

[0007] This object is achieved according to the invention by adevelopment of the known membrane modules in which, at least in thecourse of part of their substantially spiral progression to form a tubemembrane strand, at least two of the tube membranes run helically arounda common helix axis, preferably are twisted.

[0008] This invention is based on the realization that twisting theindividual tube membranes produces a profiled outer contour of theindividual tube membrane strands which, when these tube membrane strandsare spirally wound, leads to a stabilization of the membrane moduleobtained in this way, because the bounding surfaces of successivewindings of the tube membrane strands of the membrane module lie againstone another to a certain extent with positive engagement, wherebyincreased stability under loads acting on the module in the direction ofthe spiral axis is obtained. This has the effect on the one hand thatthe individual tube membranes can be prevented from departing from theirdesired position during the winding operation itself. It also makes itpossible to prevent the individual tube strands or windings from beingoffset from their desired position during handling of the membranemodules. On the other hand, the use of twisted tube membranes for theproduction of membrane modules in which the individual tube membranesrun spirally around a common axis also has the effect in most cases thatthe corresponding winding device only has to be fed one strand, whichalready has sufficient intrinsic stability, thereby simplifying thewinding operation, so that this winding operation can be carried outautomatically with control techniques of comparatively low complexity.At the same time, the number of adhesive connections between theindividual tube membranes and/or spiral windings can be reduced withoutany appreciable loss of stability, it being possible under certaincircumstances to dispense entirely with such adhesive connections or touse connections of a different kind. In the production of membranemodules according to the invention, the individual tube membranes areautomatically wound into two layers by the rope-like twisting, so that,with a two-layered winding of this type, it is also possible to dispensewith the inner reversal in the direction of lay of the individual tubemembranes by 180° which is required in the case of conventional membranemodules to obtain a two-layered winding. This has the effect ofadditionally simplifying the winding operation and the complexity of theapparatus for the winding device.

[0009] It is customary for a multiplicity of membrane modules to beoperated in parallel in a device for separating substance mixtures inthe crossflow process, only one feed line and one discharge line beingprovided for all the tube membranes. In the case of devices of thistype, maintenance work requiring the exchange of individual membranemodules can be significantly simplified if, along a first portion, thetube membranes of the module run spirally around the common axis in sucha way that the radius of curvature decreases in the direction of flow ofthe fluid flowing through the tube membranes and, along a next-followingsecond portion, they run spirally around the common axis in such a waythat the radius of curvature increases in the direction of flow. Thiscourse of the tube membranes achieves the effect that the substancemixture initially flows in the membrane module in the direction of thecommon axis and, after that, is discharged again spirally from thiscommon axis, so that both the feed line and the discharge line can bearranged outside the membrane module. This arrangement of the feed lineand discharge line makes it possible, after releasing the tubemembranes, or tube membrane strands, for individual membrane modules tobe exchanged without hindrance by the feed line and the discharge line,even when the membrane module to be exchanged is arranged between twofurther membrane modules in the direction of the common axis.

[0010] In the case of this embodiment it has proven to be particularlyfavorable if the first portion and the second portion have approximatelythe same outside diameter.

[0011] A further simplification of the production of membrane modulesaccording to the invention can be achieved while simultaneously ensuringhomogeneous filter effectiveness for all the tube membranes if all thetube membranes of the module are of the same length.

[0012] As already explained at the beginning, a particularly high filterperformance is achieved with membrane modules according to the inventionif the membrane module comprises a multiplicity of tube membranes.

[0013] These tube membranes may be realized in the membrane moduleaccording to the invention in the form of at least two strands which runin parallel, at least along part of their spiral progression, andcomprise in each case at least two tube membranes twisted in the mannerof a rope. In the case of the embodiment of the invention lastdescribed, a membrane module in which all the tube membranes are ofapproximately the same length is obtained if the strands run next to oneanother in a plane running perpendicular to the common axis, the radialsequence of the strands being reversed between the first portion and thesecond portion or the strands being arranged one behind the other in thedirection of the common axis.

[0014] Further stabilization is achieved if the membrane moduleaccording to the invention has at least two strands, each comprising atleast two tube membranes twisted in the manner of a rope, the helix axesof the strands running helically around a further common helix axis. Inthis case, it has proven to be particularly advantageous if the modulehas at least two strands, each of which comprises a plurality of tubemembranes, preferably three tube membranes. Furthermore, it has provento be favorable with regard to the desired stability of membrane modulesaccording to the invention if the tube membranes of at least one strandrun helically around a carrier of a material that is more rigid incomparison with the material of the tube membranes.

[0015] When membrane modules according to the invention are used, thefilter effect can be increased if the membranes of at least one strandrun helically around in such a way that, when there is a flow throughthe membranes, a Dean flow occurs in the membranes. The occurrence ofDean flows in devices for crossflow filtration is described, forexample, in WO 97/05946. The disclosure of this document is incorporatedin this description by express reference with regard to the arrangementof tube membranes required to obtain a Dean flow.

[0016] When producing tube membrane strands from a multiplicity of tubemembranes running helically around a common helix axis, there is theproblem that the twisting causes the individual tube membranes toundergo a different change in their length along the common helix axis.As a result, an offset of the end faces of the individual tube membranesof the individual tube membrane strands is produced along the commonhelix axis. This offset can be reduced or eliminated if, for producingmembrane modules according to the invention, use is made of a membranehelix unit with at least three tube membrane strands, each of which hasa plurality of tube membranes, preferably three tube membranes, runninghelically around a common helix axis, the helix axes of the individualstrands running helically around a further common helix axis. In afurther refinement of the invention, it is also possible for themembrane helix unit to have 3^(n) tube membrane strands, each of whichhas a plurality of tube membranes, preferably three tube membranes,running helically around a common helix axis, the individual tubemembrane strands being combined into 3^(n−1) groups of in each case3^(n−1) tube membrane strands, which run helically around a main helixaxis. In this case, each group of tube membrane strands may in turn havethree subgroups of in each case 3^(n−2) tube membrane strands, which runhelically around a common secondary helix axis.

[0017] In the case of membrane modules or membrane helix units accordingto the invention, the axial length of the individual tube membranes fora helical progression around the helix axis (length of lay) ispreferably at most 10 times the outside diameter of the tube membrane.It has been found that the separating device operates particularlyeffectively in the case of this refinement of the invention.

[0018] Furthermore, it has proven to be expedient if the radius ofcurvature of the common helix axis in the course of the spiralprogression is at least equal to 10 times the inside diameter of theindividual tube membranes. Here, too, it is true that this refinementoperates particularly effectively. With regard to the overalleffectiveness of a separating device produced using membrane modulesaccording to the invention, it has proven to be particularly favorableif the length of an individual tube membrane is at least 3000×its insidediameter.

[0019] It is preferred according to the invention for the total numberof the individual membranes in a module to be greater than 15.Compliance with this dimensioning rule leads to a particularly effectiveseparating device.

[0020] Additional stabilization of membrane modules according to theinvention can be achieved if they are surrounded at least in certainportions by a frame-shaped holder. A holder of this type not only givessaid arrangement great stability but also contributes to maintainingdistances between the individual tube membranes and/or membrane modulesthat are prescribed and advisable or even necessary with regard to theeffectiveness of the device. A further increase in stability can beobtained if at least two of the membranes of a membrane module aresecurely connected to one another. As a result, not only is greaterstability achieved, but also a prescribed distance between theindividual tube membranes is fixed.

[0021] The invention also relates to a method of producing tubemembranes running helically around a helix axis using an extrusion diefor plastic and a receiving device for the capillaries or membranes.

[0022] According to the invention, the receiving device for thecapillaries or membranes extruded from the extrusion die rotates duringthe extrusion. The method ensures particularly simple production, inparticular if two, three or four tubular membranes are extruded at thesame time. The rotational movement of the receiving device has theeffect of imparting the twisting to the membranes automatically andpermanently. Consequently, no special method step is needed to producethe twisting. Alternatively or in addition, it is also possible in thiscase to provide for the extrusion die to rotate. The same effects areachieved in this way as in the case of the refinement in which thereceiving device rotates.

[0023] The invention is explained in more detail below with reference tothe drawing, to which reference is expressly made with regard to all thedetails essential for the invention and not highlighted in thedescription. In the drawing:

[0024]FIG. 1 shows a schematic plan view from above of a separatingdevice equipped with a total of seven membrane modules according to theinvention,

[0025]FIG. 2 shows a view from the front of the separating deviceaccording to FIG. 1,

[0026]FIG. 3 shows a schematic representation of one of the membranemodules of the separating device according to FIG. 1,

[0027]FIG. 4 shows a representation of a detail of a tube membranestrand of the membrane module represented in FIG. 3,

[0028] FIGS. 5 to 9 show sectional representations of variousembodiments of tube membrane strands according to the invention,

[0029]FIG. 10 shows a schematic representation of a membrane moduleaccording to a further embodiment of the invention,

[0030]FIG. 11 shows a sectional representation of a membrane helix unitaccording to the invention,

[0031]FIG. 12 shows a sectional representation of a membrane helix unitaccording to a further embodiment of the invention and

[0032]FIG. 13 shows a schematic representation of the extrusion processaccording to the invention.

[0033] The separating device represented in FIGS. 1 and 2 substantiallycomprises a collecting container 10 and a total of seven membranemodules 20, 30, 40, 50, 60, 70 and 80. Each of these membrane modules 20to 80 is designed in the form of a flat disk. A substance mixture to beseparated with the aid of the membrane modules is fed to a feed line, towhich individual membrane modules 20 to 80 are connected. After passingthrough the membrane modules, the unseparated part of the substancemixture (the retentate) is discharged by a discharge line 14, asrepresented in FIGS. 1 and 2 by the arrows A and B. The part of thesubstance mixture that is separated by the membrane modules 20 to 80(the permeate) is collected in the container 10 and drawn off from thecontainer via a further discharge line 16, as indicated in FIG. 2 by thearrow C.

[0034] The construction of the individual membrane modules of theseparating device represented in FIGS. 1 and 2 is explained below on thebasis of FIG. 3 by way of example for the membrane module 20. Accordingto this, the membrane module 20 comprises a strand of a total of threetube membranes 22, 24, 26, which run helically around a common helixaxis W. This common helix axis W has a first portion 20 a, which runsspirally around a spiral axis S (cf. FIG. 3b) and in which the radius ofcurvature of the helix axis steadily decreases in the direction of flowA of the substance mixture flowing through the tube membranes 22, 24 and26, so that the substance mixture flows in the direction of the commonaxis S along this first spirally running portion 20 a. Following thefirst spirally running portion 20 a, the helix axis goes over into asecond spirally running portion 20 c, in the same direction of rotation.The portion 20 b has been pulled vertically apart in the drawing forbetter explanation. In practice, the spiral goes over into the secondplane in an obliquely rising manner. Along this second spiral portion 20c, the radius of curvature of the helix axis W steadily increases in thedirection of flow of the substance mixture flowing through the tubemembranes 22, 24 and 26, so that the substance mixture flows radiallyoutward with respect to the axis S along this second portion and can bedischarged with the aid of a discharge line arranged radially outsidethe spirally running portions 20 a and 20 c.

[0035] This has the overall effect of making possible the arrangement ofthe membrane module schematically represented in FIG. 3b in the form ofa flat disk, it being possible for both the feed line and the dischargeline to be arranged outside the flat disk, as represented in FIGS. 1 and2.

[0036] According to the view of a detail represented in FIG. 4 of thetube membrane strand shown in FIG. 3, the three tube membranes 22, 24and 26 are twisted with one another in a rope like manner, so that theyrun helically around a common helix axis W. In this case, this helixaxis is curved in a way corresponding to the spiral progression alongthe portion 20 a and 20 c, the radius of curvature R decreasingconstantly along the spiral portion 20 a and increasing constantly alongthe spiral portion 20 c. In the case of the embodiment of the inventionrepresented in FIG. 4, it is also ensured that the longitudinal axes ofthe individual tube membranes 22, 24 and 26 are at the same distancefrom the helix axis W.

[0037] In the case of the embodiment of the invention represented inFIG. 5, only two tube membranes 22 and 24 are twisted to form a tubemembrane strand 20. The envelope of the tube membrane strand isindicated in FIG. 5 by the circle K.

[0038] The tube membrane strand represented in FIG. 6 corresponds to thetube membrane strand explained on the basis of FIG. 4, the envelope ofthe tube membrane strand also being indicated in this drawing by thecircle K. It is also revealed particularly clearly in FIG. 6 that thelongitudinal axes 23, 25 and 27 are at the same distance from the helixaxis W in a sectional plane running perpendicular to the helix axis W.

[0039] The embodiment of the invention represented in FIG. 7 comprisesfour tube membranes 22, 24, 26 and 28 running helically around a commonhelix axis W, the envelope of the tube membrane strand formed by thesetube membranes also being indicated here by a circle K. It is also thecase in this embodiment of the invention that the longitudinal axes 23,25, 27 and 29 of the tube membranes 22, 24, 26 and 28 are at the samedistance from the helix axis W in a sectional plane runningperpendicular to the helix axis W.

[0040] The embodiment of the invention represented in FIG. 8substantially corresponds to the embodiment represented in FIG. 7, withthe additional provision of a holder 21, with which the tube membranes22, 24, 26 and 28 are held together. This frame-shaped holder 21 servesat the same time as a stabilizing device and spacer to ensure aprescribed distance between the individual windings of the spirallyrunning tube membrane strand.

[0041]FIG. 9 shows a further embodiment of the invention, in which twotube membranes are helically twisted to form a tube membrane strand. Inthe case of this embodiment of the invention, stabilization takes placewith the aid of a cross-piece 21 connecting the two tube membranes 22and 24 to one another. This cross-piece 21 crosses the helix axis W ofthe tube membrane strand and may be configured in one piece with the twotube membranes 22 and 24.

[0042]FIG. 10 shows a membrane module 120 according to the inventionwith two tube membrane strands 130 and 140. In the case of each of thesetube membrane strands 130 and 140, the helix axis W and W′ proceeds in away corresponding to the embodiment of the invention explained on thebasis of FIG. 3 along a first spirally running portion 20 a, adeflecting portion 20 b and a second spirally running portion 20 c, thestrands 130 and 140 being arranged next to one another in a planerunning perpendicular to the spiral axis and the radial sequence of thestrands 130 and 140 being reversed between the first portion and thesecond portion.

[0043] In particular with the embodiment of the invention explained onthe basis of FIG. 10, it is possible to produce disk-like windings oftube membranes with a large outside diameter and large membrane area.This is a particular advantage when constructing vertical filter systemswith large filter areas. In this case, the aim is to optimize as far aspossible in direction 1 the ratio of the overall height to the diameterof a plurality of membrane modules arranged one on top of the other,since the overall height available (height of the factory) is small inmany plants where the method is used. For this purpose, membrane moduleswith the following dimensions can be used, for example:

[0044] tube membrane inside diameter: 5.5 mm

[0045] tube membrane outside diameter: 8.5 mm

[0046] number of twisted tubes per strand: 3

[0047] number of spirally wound layers per disk: 2

[0048] disk thickness with fixing: 39 mm

[0049] inside diameter of the disk: 250 mm

[0050] individual tube length: 30 m

[0051] In this case, the inside diameter refers to the diameter of acentral opening in the module.

[0052] With these dimensions, modules which have a plurality of tubemembrane strands wound next to one another can be produced. In thiscase, a filter arrangement with an overall filter area of approximately220 m² can be produced as follows: Filter system with 220 m² of filterarea comprising 1 strand 2 strands 4 strands sprially wound disks placedtwisted with each twisted each twisted one on top of the other 3 tubeswith 3 tubes with 3 tubes Area per winding in m² 1.62 3.11 4.67 Outsidediameter in mm 635 870 1055 Number of modules arranged 135 70 47 one ontop of the other Overall structural height mm 5296 2730 1037

[0053] Finally, the further advantages of membrane modules according tothe invention can also be mentioned:

[0054] The permeate outflow between the individual wound layers isfacilitated.

[0055] The effectiveness of an intermittent permeate counter-pressurefor the dislodgment of the covering layers on the membrane surface isimproved.

[0056] The chemical cleaning of the outer side of the membrane tubes isfacilitated.

[0057] The membrane helix unit 200 represented in FIG. 11 comprises atotal of 12 tube membranes, which have been combined into three groups220, 230 and 240 with four tube membranes in each case. The tubemembranes of each of the groups 220, 230 and 240 run helically around ahelix axis W in each case, the distances between the longitudinal axesof the tube membranes of each group and the corresponding helix axis Wbeing equal in a sectional plane running perpendicular to the helixaxis. The helix axes W of the tube membrane strands formed by the groups220, 230 and 240 run helically around a main helix axis HW, all thehelix axes W being at the same distance from the main helix axis HW in asectional plane running perpendicular to the main helix axis HW. Thisarrangement of the individual tube membranes ensures that the twistingwith respect to the helix axes W or the main helix axis HW causes thesame shortening for all the tube membranes in a direction prescribed bythe main helix axis HW, so that no axial offset is produced between theend faces of the individual tube membranes.

[0058] A further embodiment of the invention, in which this axial offsetis prevented, is represented in FIG. 12. The membrane helix unitrepresented in this drawing has a total of nine tube membrane strands,each of which comprises three tube membranes running around a commonhelix axis. These nine tube membrane strands are combined into threegroups 310, 320 and 330, each of which has three tube membrane strands.In this case, the helix axes of the tube membrane strands 312, 314 and316 of the first group 310 run around a first secondary helix axis NW1,the tube membrane strands 322, 324, 326 of the second group 320 runaround a second secondary helix axis NW2 and the tube membrane strands332, 334, 336 of the third group 330 run around a third secondary helixaxis NW3. In this case it is ensured that the envelopes of the tubemembrane strands of each group lie against one another, so that all thehelix axes of the tube membrane strands of each group are at the samedistance from the corresponding secondary helix axis. The secondaryhelix axes NW1, NW2 and NW3 run helically around a main helix axis HW.In this case, it is also ensured here that the envelopes of theindividual groups of tube membrane strands lie against one another, sothat all the secondary helix axes NW1, NW2 and NW3 are at the samedistance from the main helix axis HW.

[0059] The arrangement of tube membranes represented on the basis ofFIG. 12 can be continued by combining three arrangements of the typerepresented in FIG. 12 in such a way that their main helix axes runaround a further helix axis, so that the membrane module obtained inthis way comprises a total of 27 tube membrane strands, each of whichhas three tube membranes. In this way, further arrangements with 3^(n)tube membrane strands can be put together, each of which has a pluralityof tube membranes, preferably three tube membranes, running helicallyaround a common helix axis, the individual tube membrane strands beencombined into three groups of 3^(n−1) tube membranes in each case, whichrun helically around a main helix axis, it being possible in turn foreach of these groups to have 3^(n−2) tube membrane strands, which runhelically around a common secondary helix axis, it in turn beingpossible for each of these subgroups to have 3^(n−3) tube membranestrands, which run helically around a common sub-helix axis, and so on,n designating a natural number.

[0060] An exemplary embodiment of the method of production according tothe invention is schematically shown in FIG. 13. According to this, anextrusion die 436 is provided with three outlets. Furthermore, acontainer 430 serving as a receiving device is provided. The containeris driven in a rotating manner by means of a motor 440. If the container438 rotates during the extrusion through the extrusion die 435, amembrane helix unit is obtained, made up of tubular membranes twistedwith one another, which is deposited in the container 438 in the mannerof a lasso. The rope-like twisting imparted in this way to the membranehelix unit is permanent. A special twisting operation is not required.

[0061] The features of the invention disclosed in the above descriptionand the claims and the drawing are essential, both individually and inany desired combination, for the invention to be realized in its variousembodiments.

1. A membrane module for a device for separating substance mixtures inthe crossflow process with a number of tube membranes (22, 24, 26, 28)which, at least in certain portions, run substantially spirally around acommon axis (S) to form a substantially disk-shaped module (20, 120),characterized in that, at least in the course of part of theirsubstantially spiral progression to form a tube membrane strand, atleast two of the tube membranes (22, 24, 26, 28) run helically around acommon helix axis, preferably are twisted in a rope-like manner.
 2. Themembrane module as claimed in claim 1, characterized in that, along afirst portion (20 a), the tube membranes (22, 24, 26) of the module (20)run spirally around the common axis (S) in such a way that the radius ofcurvature decreases in the direction of flow of the fluid flowingthrough the tube membranes (22, 24, 26) and, along a next-followingsecond portion (20 c), they run spirally around the common axis (S) insuch a way that the radius of curvature (R) increases in the directionof flow.
 3. The membrane module as claimed in claim 2, characterized inthat the first portion (20 a) and the second portion (20 c) haveapproximately the same outside diameter.
 4. The membrane module asclaimed in one of the preceding claims, characterized in that all thetube membranes (22, 24, 26, 28) of the module are of the same length. 5.The membrane module as claimed in one of the preceding claims,characterized in that the module has at least two strands (130, 140)which run in parallel, at least along part of their spiral progression,and comprise in each case at least two tube membranes twisted in themanner of a rope.
 6. The membrane module as claimed in one of claims 2and 5, characterized in that the strands (130, 140) run next to oneanother in a plane running perpendicular to the common axis, the radialsequence of the strands being reversed between the first portion (20 a)and the second portion (20 c).
 7. The membrane module as claimed inclaim 5, characterized in that the strands are arranged one behind theother in the direction of the common axis.
 8. The membrane module asclaimed in one of the preceding claims, characterized in that the modulehas at least two strands, each comprising at least two tube membranestwisted in the manner of a rope, the helix axes of the strands runninghelically around a further common helix axis.
 9. The membrane module asclaimed in claim 8, characterized in that the module has at least threestrands, each of which comprises a plurality of tube membranes,preferably three tube membranes.
 10. The membrane module as claimed inone of the preceding claims, characterized in that the tube membranes ofleast one strand run helically around a carrier of a material that ismore rigid in comparison with the material of the tube membranes. 11.The membrane module as claimed in one of the preceding claims,characterized in that the membranes of at least one strand run helicallyaround in such a way that, when there is a flow through the membranes, aDean flow occurs in the membranes.
 12. A membrane helix unit for adevice for separating substance mixtures with at least three tubemembrane strands, each of which has a plurality of tube membranes,preferably three tube membranes, running helically around a common helixaxis, the helix axes of the individual strands running helically arounda further common helix axis.
 13. The membrane helix unit as claimed inclaim 12, with 3^(n) tube membrane strands, each of which has aplurality of tube membranes, preferably three tube membranes, runninghelically around a common helix axis, the individual tube membranestrands being combined into 3^(n−1) groups of in each case 3^(n−1) tubemembrane strands, which run helically around a main helix axis, and nrepresenting a natural number greater than or equal to
 2. 14. Themembrane helix unit as claimed in claim 13, characterized in that eachgroup has three subgroups of in each case 3^(n−2) tube membrane strands,which run helically around a common secondary helix axis.
 15. A methodof producing capillaries or tube membranes running helically around ahelix axis, in particular for a separating device as claimed in one ofthe preceding claims, using an extrusion die for plastic and a receivingdevice for the capillaries or membranes, characterized in that thereceiving device for the capillaries or membranes extruded from theextrusion die rotates during the extrusion.
 16. A method of producingcapillaries or tube membranes running helically around a helix axis,using an extrusion die for plastic and a receiving device for thecapillaries or membranes, characterized in that the extrusion die forextruded capillaries or membranes rotates during the extrusions.